register with the phone preference service, i cant sing their praises enough. there is also a mial preference service to stop junk mail and thats an excellent service too.
http://www.mpsonline.org.uk/mpsr/
this is for the mail service, the telephone service details are on the left hand side under TPS
I use TPS, and whilst it's true it doesn't stop all of them, it does however get rid of the caller PDQ if you tell them that you've joined this service and they shouldn't be calling you.
Before I joined TPS, and whilst my little one was at the jabbering stage I used to pass the phone to her, she's waffle for ages to them- they always hung up on her though :-(